Severe Thunderstorm Warning 385, NWS Tallahassee, FL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
411 PM EDT FRI JUL 31 2015

.Near Term [Through Tonight]...

Convection is developing as expected with an early start in the FL
Big Bend. As of 18Z, we are beginning to see convection firing close
to the front across southeast AL and central GA. Outflow from these
two areas of convection will result in storms filling in in a
scattered fashion across the remainder of the forecast area. Weak
steering flow will mean that locally heavy rain will be possible. In
fact, some of our CAMs are indicating very high rainfall totals
across the eastern FL Big Bend over the next 30 hours as detailed
below.

.Short Term [Saturday Through Sunday Night]...

An area of low pressure over the northern Gulf waters will translate
northeastward across our forecast area through the weekend. This
system will bring with it plenty of Gulf moisture to our already hot
and humid local environment, bumping up precipitable water values to
2-2.5" in a swath centered along a line from Apalachicola, FL to
Savannah, GA. In addition to the forcing from the low pressure system
(and our local sea breeze front in the afternoons) and plentiful
Gulf moisture, we will also have some instability on tap with CAPE
around 1500-2000 J/kg and mid-level lapse rates around 6 C/km. With
all of these ingredients coming together, we're expecting a high
coverage of thunderstorms Saturday and Sunday, especially in north
Florida and southern Georgia.

As is typical for this time of year, some of these storms will
become strong enough to produce downbursts of damaging winds. The
main concern with these storms, however, will be heavy rainfall that
could potential cause flash flooding. Local hi-res ensemble guidance
shows a 20-40% chance of the Florida Big Bend and south-central
Georgia receiving 9"+, and even a 20% chance of exceeding 12" in the
eastern Big Bend. While 20% may not seem like a lot, it is actually
very unusual to see chances that high for rainfall amounts that high-
in other words, we have high confidence that there will be some
locally heavy rainfall totals in the Big Bend and south-central
Georgia. Therefore, we have issued a flash flood watch for tomorrow
and tomorrow night that extends from Bay county, FL to Berrien
county, GA and southeastward to allow for some buffer space for
where the heaviest rainfall might occur. North and west of the
watch, widespread rainfall totals through the weekend are expected
to be around 1-3". Where the watch is in effect, rainfall totals
will be 3-6", with isolated areas in the watch likely to receive
even higher totals.

.Long Term [Monday Through Friday]...

Above normal rain chances will continue Monday across north Florida
and south Georgia as the low pressure system slides out of our area.
By mid-week we will return to a pattern where our area sits on the
eastern edge of deep layer ridging with upper level +PV anomalies
swinging across the eastern CONUS just to our north. This pattern
along with our daily afternoon sea-breeze front rolling through will
keep rain chances around 20-40% across the area through the period.
Highs will be in the low 90s with seasonably warm lows for this time
of the year, staying in the mid 70s.

&&

.Aviation...
[Through 18Z Saturday] Scattered to numerous showers and
thunderstorms would produce brief periods of MVFR or IFR conditions
and gusty winds were they to occur at the terminals today. MVFR/IFR
conditions will possible once again overnight, most likely at ABY
and VLD. Expect an early start to the convection on Saturday as
indicated in the TAFs.

&&

.Marine...

As a low pressure system in the Gulf lifts northeastward across
north Florida and southern Georgia this weekend, the gradient will
tighten and elevate winds and seas to advisory levels. After the
system exits early next week, winds and seas will quickly return to
normal levels.

&&

.Fire Weather...

Humid conditions with above-average rain chances are expected
through this weekend.

&&

.Hydrology...

It will be a rainy weekend, with widespread areas of 1 to 3 inches
expected in southeastern Alabama and southwest Georgia. Most of the
heavy rainfall will be in the eastern Big Bend where 4 to 6 inches
rain are possible. With slow storm motion, these amounts could be
higher depending on where the thunderstorms set up. All rivers are
currently below flood stage.

&&

.Preliminary Point Temps/PoPs...

Tallahassee   76  92  74  91  75 /  60  80  60  60  50
Panama City   78  89  76  88  77 /  50  70  50  50  50
Dothan        74  93  73  93  74 /  40  60  30  40  30
Albany        75  93  73  93  74 /  50  70  30  40  30
Valdosta      74  91  74  91  74 /  50  80  50  60  40
Cross City    75  87  75  88  75 /  70  80  60  70  50
Apalachicola  78  87  77  87  78 /  60  80  60  60  50

&&

.TAE WATCHES/WARNINGS/ADVISORIES...
FL...FLASH FLOOD WATCH from Saturday morning through Sunday morning
     FOR Calhoun-Coastal Bay-Coastal Dixie-Coastal Franklin-
     Coastal Gulf-Coastal Jefferson-Coastal Taylor-Coastal
     Wakulla-Gadsden-Inland Bay-Inland Dixie-Inland Franklin-
     Inland Gulf-Inland Jefferson-Inland Taylor-Inland Wakulla-
     Lafayette-Leon-Liberty-Madison.

GA...FLASH FLOOD WATCH from Saturday morning through Sunday morning
     FOR Berrien-Brooks-Colquitt-Cook-Decatur-Grady-Lanier-
     Lowndes-Thomas.

AL...NONE.
GM...NONE.
&&

NEAR TERM...WOOL
SHORT TERM...MOORE
LONG TERM...MOORE
AVIATION...WOOL
MARINE...MOORE
FIRE WEATHER...FOURNIER
HYDROLOGY...WESTON